1. Field of the Invention
This invention relates to a volumetric control system for blower filter devices that is particularly suited for a breathing hood connection.
2. Background Art
State-of-the-art blower filter devices are characterized by the disadvantage that the flow of air supplied to the hood varies depending on how clogged the filter is. When the filter is new and clean, more air passes through the filter as is required in accordance with applicable standards in a given individual case. Similar problems occur when different filters are to be used.
The resulting disadvantages are increased power consumption and increased air throughput. Another problem arising with the gradual clogging of the filter is that it is not known when the flow of air supplied to the hood falls below the required quantity. Another problem is that the type of breathing connection that is used for the breathing hood influences the volumetric air flow.
Various types of volumetric controls have been designed to remedy this problem. EP 0 35 29 38 A2 proposes to measure the differential pressure between a measuring point in front of, and a measuring point behind, the impeller wheel of the fan and to use this signal for controlling the blower speed.
EP 0 62 10 56 A1 proposes to measure the dynamic pressure at the outlet of the blower filter device. The dynamic pressure is produced by the flow resistance of the hood and can also be used as a measure of volumetric air flow. In addition, this design features another sensor of the thermistor type in a side duct that monitors preset volumetric air flow limits and triggers an alarm signal when the air flow drops below these limits.
Fl 80606 describes a design in which the fan motor is used as a detector so that the electrical control circuit measures the power drawn by the fan motor and the effective voltage at its poles. The design uses the properties of the rotary blower, as the air volume that flows through the blower per time unit is proportional to the rotor torque, and the pressure difference is proportional to the rotational speed. This solution is improved by DE 195 02 360 A1 in that the fan output is controlled based on current and rotational speed.
Despite this comprehensive development effort, no one as yet has succeeded in keeping the volumetric air flow constant regardless of the filters and hoods that are used. Dynamic pressure measurement behind the fan or negative pressure measurement behind the fan can only be used to measure volumetric flow if the flow resistance values of the hood or filters are known. This means for practical purposes that the flow resistance values of filters and hoods have to be kept constant at narrow tolerances during production in order for these methods to work.